Reference to a publication in this section should not be construed as an indication that the publication is prior art to the present invention.
The type of immune response generated by infection or other antigenic challenge can generally be distinguished by the subset of T helper (Th) cells involved in the response. The Th1 subset is responsible for classical cell-mediated functions such as delayed-type hypersensitivity and activation of cytotoxic T lymphocytes (CTLs), whereas the Th2 subset functions more effectively as a helper for B-cell activation. The type of immune response to an antigen is generally influenced by the cytokines produced by the cells responding to the antigen. Differences in the cytokines secreted by Th1 and Th2 cells are believed to reflect different biological functions of these two subsets. See, for example, Romagnani (2000) Ann. Allergy Asthma Immunol. 85:9-18.
The Th1 subset may be particularly suited to respond to viral infections, intracellular pathogens, and tumor cells because it secretes IL-2 and IFN-γ, which activate CTLs. The Th2 subset may be more suited to respond to free-living bacteria and helminthic parasites and may mediate allergic reactions, since IL-4 and IL-5 are known to induce IgE production and eosinophil activation, respectively. In general, Th1 and Th2 cells secrete distinct patterns of cytokines and so one type of response can moderate the activity of the other type of response. A shift in the Th1/Th2 balance can result in an allergic response, for example, or, alternatively, in an increased CTL response.
It has been recognized for some time that a Th1-type immune response can be induced in mammals by administration of certain immunomodulatory polynucleotides. The immunomodulatory polynucleotides include sequences referred to as immunostimulatory sequences (“ISS”), often including a CG dinucleotide. See, e.g., PCT Publications WO 98/55495, WO 97/28259, U.S. Pat. Nos. 6,194,388 and 6,207,646; and Krieg et al. (1995) Nature 374:546-49. For many infectious diseases, such as tuberculosis and malaria, Th2-type responses are of little protective value against infection. Protein-based vaccines typically induce Th2-type immune responses, characterized by high titers of neutralizing antibodies but without significant cell-mediated immunity. Moreover, some types of antibody responses are inappropriate in certain indications, most notably in allergy where an IgE antibody response can result in anaphylactic shock.
In view of the need for improved methods of immunotherapy, a need exists for identification of compounds useful for modulation of an immune response.